Tell HN: I did it (thank you)
Long story short I didn't jump ship, I stuck with the job, but ended up pregnant with #2 who was born in May 2012. I realised that I'd be returning to work to pay for childcare and so took the leap into self-employment.
I now work from home around 2 kids(!) - a 3.5yr old and nearly 1yr old - and am coming up to the end of my first year in business. It's hard, REALLY hard at times, but I am booked up solid months in advance and have made more in the past few months than I did in my job despite working around 8-12 hours less per week on average. My partner has been able to go down to part time hours so that he can spend more time with the kids which is benefiting us both.
I have launched a website for other UK work at home parents offering advice and support, and this is growing steadily month on month.
Anyway, I just wanted to say thank you. I would not have had the guts to go through with this or indeed known the realities of what to expect if I'd not got such a fantastic range of responses to that original thread.